55 ILCS 5/1-4016
The territory which constituted the petitioning county shall continue and remain until the next apportionment of the state for congressional, legislative or judicial purposes part of the same congressional district, of the same senatorial district, of the same judicial grand division, of the same judicial appellate district and of the same judicial circuit that it constituted part of at the date of the proclamation aforesaid; and at any election, where the territory that had constituted the respective counties before said proclamation is in different districts, the county clerk of the adjoining county shall keep separate the votes polled in the territory constituting the respective counties before said proclamation, until the next apportionment aforesaid, and shall report and return the same separately to the Secretary of State.
55 ILCS 5/1-4016